1. The present invention relates to a light-modulation element composed of liquid crystals, which is applied to a display, a light-shutter and the like, and further to a production method thereof.
2. Description of the Invention
Recently a liquid crystal device of polymer-dispersion type constituted of a polymer-liquid crystal composite has been paid attention to as a display from the viewpoints:its easy production without an orientation treatment; a clear display achieved by use of no polarizing plate.
In general a liquid crystal device of polymer-dispersion type is produced by forming a liquid crystal-polymer composite composed of liquid crystals dispersed and kept in a polymer matrix between a pair of electrodes. The refractive index for ordinary rays in the liquid crystals is adjusted to be almost as same as that of the polymer matrix. In more detail, liquid crystal molecules are oriented almost parallel to an interface between the liquid crystal and the polymer matrix when no voltage is applied. Under such a state when light rays are put into the device perpendicularly to the substrate (electrode), the light rays scatter at the interface because the refractive index of polymer matrix is different from that of liquid crystal. When a voltage is applied between the electrodes, liquid crystal molecules are lined up in lines almost perpendicular to the electrodes in the case of nematic liquid crystal having positive dielectric anisotropy. In this case, the refractive index of polymer matrix becomes almost as same as the refractive index for ordinary ray in the liquid crystal, so that light rays transmit without scattering. The above characteristics make it possible to give a light-shuttering element.
However because a polarizing plate is not used, the contrast between turning on and turning off states is liable to be inferior compared with that of a conventional display which utilizes a polarizing plate. The contrast defined in this case is a ratio of light flux reaching a light-receiving portion under voltage-applied conditions to light flux reaching a light-receiving portion under no voltage-applied condition. As the contrast is higher, image quality becomes better.
A display mode of a liquid crystal device of polymer-dispersion type is controlled by alternating a light-scattering state and a light-transmitting state. These states depend on whether an electric field is formed or not. It is ideal that when no voltage is applied, all light rays scatter completely and almost no transmitted-light reaches a light-receiving portion. However in practice a number of light rays transmit without scattering. Therefore the improvement of light-scattering properties in the no voltage-applied state can lead to the improvement of performance of the display mode, resulting in high contrast and clear display.
A liquid crystal device of polymer-dispersion type is supposed to show three times the brightness of a conventional twisted nematic (referred to as "TN" hereinafter) mode in a voltage-applied state because a polarizing plate is not used. In practice such a sufficient brightness can not be often achieved because a polymeric material layer absorbs transmitted light and the refractive index of the liquid crystal is not coincident with that of the polymer matrix.
There is also a problem such that a liquid crystal device of the polymer-dispersion type requires a high driving voltage compared with a display device of TN mode. Therefore a driving IC available in the market can not be applied to the liquid crystal device of polymer-dispersion type.
In a conventional liquid crystal device of the polymer-dispersion type, light-scattering properties are determined by the difference between refractive index of liquid crystal and that of polymer. However, a large difference of refractive indexes can not be obtained practically from the viewpoint of physical properties of liquid crystal materials. Therefore scattering intensity is limited. High brightness can be achieved but contrast is much inferior to that of TN mode.
Further there is a problem such that polymeric materials absorb incident light in a voltage-applied state (i.e. a light transmitting state) to deteriorate light transmittance.
Since a conventional liquid crystal device of polymer-dispersion type has a high content of polymeric material in a polymer-liquid crystal composite, an effective voltage applied to the liquid crystal is low. Therefore a high driving voltage is applied. For example, liquid crystal devices of polymer-dispersion type are disclosed in prior arts such as Japanese Patent National Publication Sho 61-502128, Japanese Patent Laid-Open Hei 02-198365 and Japanese Patent Laid-Open Hei 04-106523 have respectively a high content of a polymeric material and require a driving voltage higher than 25 V. Therefore a driving IC available in the market can not drive the above liquid crystal device of polymer-dispersion type.